Regarding a liquid crystal display device, based on the displaying mode of the liquid crystal, it can be classified into the types of PC (phase change), TN (twisted nematic), STN (super twisted nematic), ECB (electrically controlled birefringence), OCB (optically compensated bend), IPS (in-plane switching), VA (vertical alignment) and the like. Based on the driving mode of the device, it can be classified into the PM (passive matrix) type and the AM (active matrix) type. PM is classified into the static type, multiplex type and so forth. AM is classified into TFT (thin film transistor) type, MIM (metal insulator metal) type and so forth. The types of TFT comprise amorphous silicon and polycrystal silicon. The latter is classified into a high temperature type and a low temperature type according to the manufacturing process. Based on the type of the light source, the liquid crystal display device is classified into a reflection type utilizing a natural light, a transmission type utilizing a backlight and a semi-transmission type utilizing both the natural light and the backlight.
TABLE 1The general characteristics of the composition and the AM deviceThe general characteristics The general characteristics No.of the compositionof the AM device1Temperature range of a nematicUsable temperature range is phase is wide.wide.2Viscosity is small.Response time is short.3Optical anisotropy is suitable.Contrast ratio is large.4Dielectric anisotropy isThreshold voltage is low, electricpositively or negatively large.power consumption is small, and a contrast ratio is large.5Specific resistance is large.Voltage holding ratio is large, and a contrast ratio is large.6Stable to ultraviolet light andService life is long.heat.
Liquid crystal materials are required to have an appropriately high dielectric anisotropy, optical anisotropy and a good low-temperature intersolubility and heat stability. In addition, the liquid crystal materials should have a low viscosity, a short response time, a low threshold voltage and a high contrast ratio. The various performance index of the composition will be further explained based on a commercially available crystal liquid display device. The temperature range of a nematic phase relates to the working temperature range of the device. A desirable upper limit temperature of the nematic phase is 70° C. or more, and a desirable lower limit temperature of the nematic phase is −10° C. or less. The viscosity of the composition relates to the response time of the device. A short response time of the device is desirable for displaying a moving image in the device. Accordingly, a small viscosity of the composition is desirable, and a small viscosity of the composition at a low temperature is more desirable.
The optical anisotropy of the composition relates to the contrast ratio of the device. In order to maximize the contrast ratio of the liquid crystal display device, the product value (Δn*d) of the optical anisotropy (Δn) of the liquid crystal composition and the thickness (d) of the liquid crystal layer can be designed to be a fixed value. A suitable product value depends on the kind of operation mode. In a device having a TN mode, a suitable value is approximately 0.45 μm. In this case, a composition having a large optical anisotropy is desirable for a device having a small thickness of the liquid crystal layer.
The contrast ratio of a liquid crystal display device can be increased by including a liquid crystal composition having a larger optical anisotropy.
A liquid crystal display device containing a liquid crystal composition having a large absolute value of dielectric anisotropy can decrease the base voltage value and the driving voltage, and can further decrease the electric power consumption. There is an inverse relation between the absolute value of dielectric anisotropy and the driving voltage. A liquid crystal display device with a liquid crystal composition having a low driving voltage value can effectively decrease the power consumption of the display, and therefore, consumables, such as mobile phones, tablet PCs and other portable electronic products, may have a longer endurance.
The risk of the residual image occurrence can be reduced for a liquid crystal display device containing a liquid crystal composition having a higher voltage holding ratio, and therefore, the service life of the liquid crystal display device can be prolonged.
It is usually difficult for a single liquid crystal compound to present its characteristics, thus a composition is normally prepared by mixing the single compound with a variety of other liquid crystal compounds. Although the liquid crystal compositions in the prior art can present better characteristics, these liquid crystal compositions (e.g., liquid crystal compositions disclosed in CN101796162A and CN1475547A) all have the following disadvantages in varying degrees: optical anisotropy is not large enough, dielectric anisotropy is relatively low, clearing point is relatively low, viscosity is relatively high, and voltage holding ratio is low, which directly result in that the liquid crystal devices using these liquid crystal compositions have a low contrast ratio, a frequent residual image occurrence, a large electric power consumption and other defects.
Therefore, there is a need for a liquid crystal composition with at least one characteristic among the characteristics of a large optical anisotropy, a large dielectric anisotropy, a low threshold voltage, a high voltage holding ratio, such that the liquid crystal device may have a higher displaying contrast ratio and the like.